Most everyone experiences the joy of riding a bike at one point in their lifetime and many learn to ride at a very young age. Because a young child can master its basic principles, the act of riding a bike itself appears very simple. The physics behind the exhilarating act itself, however, are anything but. The cyclist needs to overcome numerous types of forces acting on the properties of balancing, steering, braking, accelerating, suspension activation, vibration and many other bicycling characteristics. Moreover, many of the forces in each physical realm are open to change and depend on their surrounding environment and/or forces from other properties, which adds several orders of complexity.
To consider the complexities of a bike as a whole becomes somewhat overwhelming. Nevertheless, each force that acts on the physical aspects or properties associated with a bike and its rider can be broken down into smaller, more manageable pieces. For example, if we consider a cyclist or rider and her bike as a single system, the forces that act on that system and its components can be roughly divided into two groups: internal and external forces. Internal forces are mostly caused by the rider and the rider's interaction with the bike (e.g., by bicycle component friction). External forces, on the other hand, are due to gravity, inertia, contact with the ground, and contact with the atmosphere.
While the internal forces can have a significant impact on bicycle performance, most any bicycle racer will agree that the largest resistance comes from the induced external force of the bicycle's movement through the air. As a rider attempts to move faster, the atmospheric drag and crosswind forces become greater, which in turn requires the rider to expend greater energy to overcome them. Thus, these forces become an important consideration in bike designs, especially in the areas of bicycle racing and triathlons.
One of the major sources of these dynamic atmospheric forces results from the flow of air over and around the bicycle wheels. Over the years, many have attempted to reduce the drag in wheels through the use of a “solid” or “disc” wheel, which are wheels without spokes. Such disc shape alleviates the drag caused by the movement of air over the spokes and over and around the wheel rim; however, such rims suffer from stability control caused from the other aerodynamic force of crosswind. More specifically, as wind forces perpendicular to the disc surface increase, an increased wind-loading force is transmitted from the disc surface to the bicycle handlebars. This requires the rider to apply a control force to the handlebars that varies as the wind-loading changes. Additionally, the force exerted by a sudden crosswind can destabilize the bicycle and rider; resulting in a need for forward speed reduction to regain stability.
Until recently, cyclists have been forced to choose either traditional spoked wheels with their inherent drag component or solid wheels with their inherent crosswind disadvantages. Recent attempts to reconcile these two types of wheels have led to a compromise wheel design. This design employs a limited number of solid spokes in conjunction with the bicycle rim. These wheels resemble a solid wheel with large “cutouts” in the disc to minimize the crosswind effects. Despite these efforts, the compromise designs can still suffer from objectionable crosswind, wind-loading, drag and otherwise do not include optimum aerodynamic or structural characteristics.